Concurrently filed copending patent application Ser. No. 486,470, which is entitled "Solar Energy Converter Using Surface Plasma Waves", the subject matter of which is hereby incorporated by reference, describes surface plasmon properties and coupling techniques suitable for solar energy conversion. Reference is made to the patents and other materials cited in the above mentioned patent application for discussion of other prior art techniques of possible interest.
Conventional semiconductor solar cells are covered by U.S. Pat. No. 4,219,830 to Gibbons. The semiconductor solar cell described in this patent has an electric field directed from the surface of the cell towards the center of the body such that minority carriers tend to drift away from the surface of the cell body. The cell has a graded p-type layer extending from all surfaces into the body or interior of the cell. An electric field is generated by the graded surface to the body regions, and the effect of this field forces electrons to drift to the interior of the cell so that holes drift to the heavily doped cell surface. As pointed out in copending application Ser. No. 486,470, over 50% of the incident power on such a solar cell will be wasted because of the mismatch between the broad solar spectrum and the monoenergetic conduction electrons which transport energy within the cell.
Sabnis et al U.S. Pat. No. 4,206,002 describes a solar cell having a gradually changing chemical composition which produces a gradually changing bandgap within the cell. Different sections of the cell have different bandgaps to enable the cell to absorb many different ranges of photon energy. The cell can convert several photon wavelengths or energy ranges into electrical energy.
Lamorte U.S. Pat. No. 4,179,702 discloses a monolithic cascade solar cell structure in which the junctions within the first and second energy converting layers are homojunctions, but the third layer provides a homojunction operating as a tunnel junction to connect the layers together. The bandgap of the third layer forming the tunnel junction is greater than the bandgap of the first layer so that the third layer is optically inactive. Photons passing through the first layer are not converted into electrical energy and pass unattenuated to the bottom or second layer.